1. Field of the Invention
The present invention relates to the manufacture of high performance VLSI semiconductor chips in general and more particularly to a lift-off method for making metal contact studs between first level metal and the desired regions of polyimide-filled deep trench-isolated devices, typically bipolar NPN transistors.
2. Prior Art
Recent developments of advanced semiconductor devices, such as bipolar transistors, have been executed using polysilicon base contacts in conjunction with shallow junctions. There is also a strong tendency to extensively use the polyimide-filled deep trench isolation for denser integration.
Although the polysilicon base contact reduces Ccb and Rb, it adds additional topology steps (0.7 .mu.m) to the master slice structure. Moreover, standard manufacturing techniques of polyimide-filled deep trench isolated devices generally reduce trench widths to a minimum. With conventional metallurgical schemes using the so called first level metal technique, this results in increased metal land capacitance (first level metal to substrate). It also results in primary technology problems of metal thinning (edge coverage) at or over abrupt trench isolation steps and, parasitic FETs which can occur when first level metal conductors lie over thin field dielectrics.
An alternate well-known metallurgical technique consists in employing the stud technique. There is abundant literature concerning that subject, e.g. an article entitled "High current internal contact structure for integrated circuits" by C. E. Benjamin and published in the IBM Technical Disclosure Bulletin, Vol. 19, No. 10, March 1977, page 3732.
According to that article, metal studs are first formed over the individual contact holes in the thermal silicon dioxide layer masking a silicon wafer, before the first level metal is deposited using different photolithography processes.
However, none of the stud contacting art known to the present applicant is directed to semiconductor devices manufactured according to the polyimide-filled deep trench isolation technology. Standard oxygen Reactive Ion Etching (RIE) cannot be used directly with that technology, because the polyimide fill is attached. Furthermore, forming the contact studs using metal RIE or wet sub-etch processes, result in residual metal rails (caused by the anisotropy of this type of etching) or excessive process bias, respectively.
In addition, the problem of implementing metal contact studs overlapping the contact regions and the surface of the trenches adjacent thereto, in order to increase density by alleviating misregistration concerns, has not been addressed at all up to now.